linux-kernel - [tip: x86/alternatives] x86/alternative: PAUSE is not a NOP

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Message-ID: <168676457432.404.2368644491901077612.tip-bot2@tip-bot2>
Date:   Wed, 14 Jun 2023 17:42:54 -0000
From:   "tip-bot2 for Peter Zijlstra" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: x86/alternatives] x86/alternative: PAUSE is not a NOP

The following commit has been merged into the x86/alternatives branch of tip:

Commit-ID:     2bd4aa9325821551648cf9738d6aa3a49317d7e5
Gitweb:        https://git.kernel.org/tip/2bd4aa9325821551648cf9738d6aa3a49317d7e5
Author:        Peter Zijlstra <peterz@...radead.org>
AuthorDate:    Wed, 14 Jun 2023 16:35:50 +02:00
Committer:     Borislav Petkov (AMD) <bp@...en8.de>
CommitterDate: Wed, 14 Jun 2023 19:02:54 +02:00

x86/alternative: PAUSE is not a NOP

While chasing ghosts, I did notice that optimize_nops() was replacing
'REP NOP' aka 'PAUSE' with NOP2. This is clearly not right.

Fixes: 6c480f222128 ("x86/alternative: Rewrite optimize_nops() some")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/linux-next/20230524130104.GR83892@hirez.programming.kicks-ass.net/
---
 arch/x86/kernel/alternative.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index bbfbf7a..a7e1ec5 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -169,9 +169,12 @@ void text_poke_early(void *addr, const void *opcode, size_t len);
  */
 static bool insn_is_nop(struct insn *insn)
 {
-	if (insn->opcode.bytes[0] == 0x90)
+	/* Anything NOP, but no REP NOP */
+	if (insn->opcode.bytes[0] == 0x90 &&
+	    (!insn->prefixes.nbytes || insn->prefixes.bytes[0] != 0xF3))
 		return true;
 
+	/* NOPL */
 	if (insn->opcode.bytes[0] == 0x0F && insn->opcode.bytes[1] == 0x1F)
 		return true;